An image forming apparatus such as a digital color copying machine converts RGB (red, green, blue) image data read out by an image input apparatus such as a scanner into four colors of CMYK (cyan, magenta, yellow, black) and outputs the colors. In a case where the image forming apparatus copies an original document which includes texts and halftone dot images, in order to improve image quality of black texts of the document, there is performed a process in which edges of black texts are extracted and filtered so that the edges are enhanced, or a process in which amounts of CMY are dropped while an amount of black is increased before outputting the image.
However, in a case where the image input apparatus such as a scanner reads an image, there is a possibility that reading position of a CCD sensor is aberrant due to mechanical vibration or other cause (chromatic aberration) and color fringing (out of color registration) in which edges of black texts are stained with colors is occurred. In an area where the color fringing occurs, CMY is unbalanced. Consequently, in the area, if there is performed a generally-performed under color removal process based on a minimum value of CMY (a value having the lowest level out of respective levels of C density signal, M density signal, and Y density signal), then an enough amount (density) of black is not generated, which raises such a problem that small black texts are difficult to be read or black lines are stained with colors.
Therefore, the applicant of the present application proposed in Document 1 a method in which: an input image is subjected to a segmentation process and text areas are extracted, the text areas are classified into a black text area, a color fringing area, and other area, an amount of black generation is controlled with respect to each area, and as for pixels of the color fringing area, an amount of black generation is determined based on an average value between a maximum value and a minimum value of image data having a plurality of color components (CMY).
Further, Document 2 discloses a technique in which: differences between two colors of different combinations of three primary colors read out from a color original document are calculated, a maximum value of the differences is obtained for each pixel, the maximum value is compared with a threshold value so that the pixel is classified as a chromatic area or an achromatic area, and the pixel classified as the achromatic area is formed only with a black toner.
(Document 1)
    Japanese Unexamined Patent Publication No. 223915/2001 (Tokukai 2001-223915; published on Aug. 17, 2001)(Document 2)    Japanese Unexamined Patent Publication No. 184075/1995 (Tokukaihei 7-184075; published on Jul. 21, 1995)(Document 3)    Japanese Unexamined Patent Publication No. 264701/2003 (Tokukai 2003-264701; published on Sep. 19, 2003)
However, the above-mentioned conventional techniques have such a problem that color fringing and a white spot (peripheries of black texts or black lines are represented with white or gray) cannot be overcome at the same time.
Table 1 describes characteristics of the conventional techniques (color fringing at a time of chromatic aberration, a white spot at a time of chromatic aberration (white spot generated around texts and lines on a chromatic page background and on halftone dots), and characteristics of a circuit (hardware) configuration). Note that in Table 1, characteristics of the conventional techniques are represented from a less preferable state to a more preferable state by using signals of X, Δ, ◯, ⊙.
TABLE 1ColorWhiteCircuitfringingspotconfigurationConventional generalX⊚⊚technique (UCR based onmin (CMY)Technique of Document 1◯~XΔ~XX(process is switched byrecognizing blurred area)Technique of Document 2⊚XΔ(only black is processed)
As illustrated in Table 1, in the conventional general technique, namely, in a technique in which UCR based on a minimum value of CMY is performed, a process of UCR is not switched with respect to each area, so that it is unnecessary to provide a new circuit for switching the process, and a white spot does not occur. However, as described above, when chromatic aberration occurs, a color component (color fringing) is clearly represented.
Further, the technique of Document 1 has such a problem that it is difficult to exactly discriminate a color fringing area in a current technique standard and therefore image quality is liable to be deteriorated due to misdiscrimination of the color fringing area. Namely, color fringing does not occur uniformly (color is greatly aberrant at one time and color is slightly aberrant at another time) and therefore a color fringing area is not recognized with high exactness in the current technique standard. Consequently, an original document on which a color line and a black line are actually written or an image in which a periphery of a text or a line is represented with colors is liable to be wrongly recognized as a color fringing area. In particular, a black edge (a periphery of a black text or a black line) on a colored paper or a halftone dot image is liable to be wrongly recognized, which deteriorates image quality. Further, Document 1 has such a problem that a circuit is necessary for recognizing a color fringing area and therefore circuit configuration becomes complex.
Further, the technique of Document 2 has such a problem that a periphery of a black text or a black line is represented with white or gray (white spot). Namely, in the technique of Document 2, a black text having low density is recognized as an achromatic area and represented only with black. Consequently, it is possible to prevent color fringing, but a page background component and a halftone dot component are also formed only with black, so that a white spot is generated. Further, a switching circuit for changing all processes of CMYK in accordance with a discriminated area is necessary, which complexes circuit configuration.